1. Field of the Invention:
This invention relates to leuco dyes. More specifically, this invention relates to novel blocking groups for the preparation of blocked leuco dyes, particularly cyan, magenta, and yellow blocked chromogenic leuco dyes, that are suitable for use in photothermographic imaging systems.
2. Background Art:
Silver halide-containing, photothermographic imaging materials (i.e., heat-developable photographic elements) processed with heat, and without liquid development, have been known in the art for many years. These materials, also known as "dry silver" compositions or emulsions, generally comprise a support having coated thereon: (a) a photosensitive material that generates elemental silver when irradiated; (b) a non-photosensitive, reducible silver source; (c) a reducing agent for the non-photosensitive, reducible silver source; and (d) a binder. The photosensitive material is generally photographic silver halide that must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that elemental silver (Ag.degree.) is a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions. For example, catalytic proximity can be accomplished by partial metathasis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075); by coprecipitation of silver halide and the reducible silver source material (see, for example, U.S. Pat. No. 3,839,049); and other methods that intimately associate the photosensitive photographic silver halide and the non-photosensitive, reducible silver source.
The non-photosensitive, reducible silver source is a material that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further processed in order to produce a visible image. The visible image is produced by the reduction of silver ions, which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, i.e., the latent image. This produces a black and white image.
As the visible image is produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable in order to reduce the cost of raw materials used in the emulsion. One method of attempting to increase the maximum image density in photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming materials in the emulsion and producing color images.
A number of methods have been proposed for obtaining color images with dry silver systems. Such methods include, for example, incorporating dye-forming coupler materials into the dry silver systems. For example, known color-forming dry silver systems include: a combination of silver benzotriazole, a magenta, yellow or cyan dye-forming coupler, an aminophenol developing agent, a base release agent such as guanidinium trichloroacetate, and silver bromide in poly(vinyl butyral); and a combination of silver bromoiodide, sulphonamidophenol reducing agent, silver behenate, poly(vinyl butyral), an amine such as n-octadecylamine, and 2-equivalent or 4-equivalent cyan, magenta or yellow dye-forming couplers.
Color images can also be formed by incorporation of leuco dyes into the emulsion. A leuco dye is the reduced form of a color-bearing dye. It is generally colorless or very lightly colored. Upon imaging, the leuco dye is oxidized, and the color-bearing dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye enhanced silver image can be produced, as shown, for example, in U.S. Pat. Nos. 4,187,108; 4,374,921; and 4,460,681.
Multicolor photothermographic imaging elements typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsion layer. Photothermographic elements having at least two or three distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4,021,240 and 4,460,681. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are well know in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747; and Research Disclosure, March 1989, item 29963.
When the reactants and reaction products of photothermographic systems remain in contact after imaging, several problems can result. A common problem is the instability of the image following processing. The photoactive silver halide still present in the developed image can continue to catalyze development of metallic silver even under room light, thereby causing a strong increase of fog after development. This development of fog is also increased by exposure to oxygen, which causes the oxidation of leuco dyes. For example, U.S. Pat. Nos. 4,670,374 and 4,889,932 describe photothermographic materials containing oxidizable leuco phenazine, phenoxazine, or phenothiazine dyes useful for color photothermographic images, which are subject to air oxidation causing fog after development. In addition, for photothermographic systems containing leuco dyes the resulting prints tend to develop color in unimaged background areas during storage. This "background stain" is caused by slow reaction between the leuco dye and an oxidizing agent.
Another problem for photothermographic systems containing leuco dyes is the lack of stability of the leuco dye before exposure. In fact, in many cases, it is not possible to obtain any images because the leuco dye reacts in a non-imagewise manner before exposure. The consequence of this non-imagewise reaction is the absence of differences in density between the imaged and non-imaged areas. This means that there is no difference in the development between the parts that should have produced an image and the parts that should not have produced an image.
Thus, there exists a need for useful leuco dyes for photothermographic materials that are stable enough not to be oxidized by contact with air or by simple heating, and which limit fog formation after development. One approach by which these criteria can be met is through the use of blocking or modifying groups that assist in providing stability to the leuco dyes and in decreasing their diffusibility. The blocking groups must also be easily removed from the leuco dye during processing without the use of highly basic solutions or other reagents that cause adverse effects on the other components of the imaging construction. Furthermore, the blocked leuco dyes must react imagewise to provide a good dye image.
British Patent No. GB 1,417,586 describes oxichromic compounds containing a reduced azomethine linkage that can be blocked with a group that prevents oxidation of the nitrogen atom of the azomethine linkage. Such compounds produce a chromophore useful in color photographic systems in silver halide transfer materials upon chromogenic oxidation and upon removal of the blocking group by hydrolysis in alkaline solution. European Patent Application No. 35,262, and PCT Patent Application No. WO 90-00978 describe, respectively, non-silver copy materials and non-silver heat-sensitive materials both having leuco dyes with the same --SO.sub.2 -- protecting group. These leuco dyes are useful in heat-sensitive materials; however, they are not useful in photothermographic materials because they do not react imagewise to give a dye image. In fact, when the material containing such leuco dyes is exposed and developed according to the usual process for photothermographic materials, it does not present any difference in density between the imaged and non-imaged areas.
There are relatively few blocking groups that can be used to stabilize leuco dyes, particularly chromogenic leuco dyes, in photothermographic materials. A few examples include --C(O)--NH--C.sub.6 H.sub.4 --N(CH.sub.3).sub.2, --C(O)--NH--C.sub.6 H.sub.4 --N(CH.sub.2 CH.sub.3).sub.2, --C(O)--NH--(CH.sub.2).sub.3 --CH.sub.3, --C(O)--NH--CH.sub.2 --CH=CH.sub.2, --C(O)--C.sub.5 H.sub.11, --C(O)--OC.sub.3 H.sub.7, --C(O)--NH--C.sub.6 H.sub.4 --CH.sub.3, --C(O)--NH--C.sub.2 H.sub.5, --C(O)--NH--C.sub.6 H.sub.5, --C(O)--NH--C.sub.3 H.sub.7, --C(O)--C.sub.6 H.sub.5, --C(O)--C.sub.6 H.sub.4 --OH. Also, chromogenic leuco dyes having various protecting groups such as carbamoyl are described in Applicants' Assignee's copending application Ser. Nos. 07/939,093 (filed Sep. 2, 1992 which issued as U.S. Pat. No. 5,330,864 on Jul. 19, 1994) and 08/161,900 (filed Dec. 3, 1993). Because of the limited number of blocking groups, there are limited numbers of photothermographic systems. Thus, a need exists for more leuco dye blocking groups, particularly blocking groups that are easy to remove during development and allow for the formation of sufficient differences in density between imaged and non-imaged areas.